Semiconductor image sensors are used for sensing light. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupled device (CCD) sensors are widely used in various applications such as digital still camera or mobile phone camera applications. These devices utilize an array of pixels in a substrate, including photodiodes and transistors, that can absorb radiation projected toward the substrate and convert the sensed radiation into electrical signals.
A back side illuminated image sensor (BSI) device is a type of image sensor device. Fabrication of the BSI device typically requires a thinning process that reduces the thickness of the substrate. However, if the substrate is too thin, the BSI device may not be able to absorb radiation efficiently, particularly with respect to radiation that has a long wavelength. In other words, the sensitivity of the BSI device may be degraded as the thickness of the BSI substrate is reduced. On the other hand, if the substrate is not sufficiently thin, cross-talk between neighboring pixels may occur, which reduces image quality. Hence, existing BSI devices may require undesirable tradeoffs between light sensitivity and image quality.
Therefore, while existing methods of fabricating BSI image sensors have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect.